Add script to generate XML for Endless OS, and add 3.6.2

Closes #7 (closed). I based the template on the existing eos-3.4.xml.in file.

I have some questions:

Are <upgrades> and <derives-from> mandatory? This would imply that I should also generate XML for eos-3.0 through eos-3.5 – I guess that's easy enough to implement.

We (mostly) follow a tick-tock-tock-tock-tock-tock release pattern where every 6 months we release a new 3.X.0 major version, and then we make a point release in each intervening month. Our point releases are not strictly bugfixes as is common in other distros, though the major releases do contain more dramatic changes (eg new GNOME versions). I've followed the example used for (eg) Fedora and generated eos-3.6.xml.in, and we'd send another MR to update it in 2½ weeks when we release 3.6.3. Is that OK?

Right now, in Boxes, there is no logo for Endless OS in the Download an OS → Show More… list. Is that some missing metadata here? Or do we need to submit our logo to some other repo?